1. Field of the Invention
The present general inventive concept relates to a developing unit to supply a toner to an electrophotographic image forming apparatus and contributes to developing an image and an image forming apparatus using the same, and more particularly to a developing unit having a regulating blade with an enhanced structure to regulate a supplied toner layer and image forming apparatus using the same.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus forms an electrostatic latent image by scanning light onto a photosensitive medium charged to a predetermined electric potential, develops the electrostatic latent image by toner of a predetermined color, transfers the developed image to a printing medium, and fixes the transferred image to produce a printed image.
The electrophotographic image forming apparatus can be divided into a wet-type and dry-type electrophotographic image forming apparatus by its toner type, and the dry-toner type image forming apparatus can be divided into a two-components developer type and a one-component developer type depending on whether a carrier is used in the developer. The two-component developer type image forming apparatus uses a two-component developer which includes a carrier and a toner mixed in a powder state, respectively. The one-component developer type image forming apparatus uses a one-component developer which includes only the toner without the carrier.
The image forming apparatus includes a photosensitive medium which forms an electrostatic latent image by being charged and exposed to light, and a developing unit which supplies the photosensitive medium with the toner to form the image.
FIG. 1 is a partial sectional view illustrating a conventional developing unit employed in the one-component developer type image forming apparatus. As illustrated in FIG. 1, the conventional developing unit includes a housing 3 filled with toner T of a predetermined color, a developing roller 5 and a supplying roller 7 both supplying a photosensitive medium 1 with the toner T, and a regulating blade 9 regulating a toner layer formed on the developing roller 5.
The supplying roller 7 contacts with the developing roller 5 and rotates in the same direction in which the developing roller 5 rotates. Therefore, the toner in the housing 3 is friction-charged at a nip which is formed between the developing roller 5 and the supplying roller 7, and is supplied to the developing roller 5 by a potential difference between bias-voltages applied to the developing roller 5 and the supplying roller 7.
The developing roller 5 is coupled to the housing 3 to be rotatable opposite to the photosensitive medium 1. The developing roller 5 supplies the photosensitive medium 1 with the toner and develops an image on an area of the electrostatic latent image, which is formed at a surface of the photosensitive medium 1, by its potential difference with the photosensitive medium 1.
The regulating blade 9 contacts with the developing roller 5 and regulates the toner layer, which is formed by the toner supplied to a surface of the developing roller 5, to have a predetermined thickness. To this end, the regulating blade 9 includes a free end part 9a which is bent accordingly by an external pressure, and a fixed end part 9c which fixes the free end part 9a to the housing 3.
In this case, for the toner layer to uniformly have a thickness which is about 1.5 times larger than the average diameter of a toner particle, a tip pressure which acts on a tip part 9b where the free end part 9a contacts with the developing roller 5 is required to be maintained beyond a predetermined value. On the other hand, the tip pressure is influenced by a change of a regulating angle θ1 which is defined by a surface of the free end part 9a and a tangent line formed by a contact between the free end part 9a and the developing roller 5 at the tip part 9b. That is, under the same conditions, the larger the regulating angle θ1 becomes, the smaller the tip pressure is required to regulate the thickness of the toner layer formed on the developing roller 5.
If the regulating angle θ1 becomes increased to lower the tip pressure, the height H of the housing 3 becomes larger. Therefore, an overall height of the developing unit may be larger if the developing unit of a structure as illustrated in FIG. 1 is provided plurally to correspond to each color and if each of the developing units is provided at a side of the photosensitive medium 1 of a drum type thus forming a color image forming apparatus of a multi-path type. Therefore, according to the above conventional developing unit, the overall height of the image forming apparatus becomes larger. Also, a diameter of the photosensitive drum should be larger if the drum type photosensitive medium 1 is employed, or otherwise a belt type photosensitive medium 1 should be employed.
On the other hand, if the regulating blade 9 is provided so that the regulating angle θ1 may be about 24° to miniaturize the overall size of the developing unit, the toner layer may have a uniform thickness when the tip pressure acting on the tip part 9b where the free end part 9a contacts with the developing roller 5 is maintained beyond a predetermined value, for example, 50 gf/cm.
However, if the tip pressure of the regulating blade is maintained beyond 50 gf/cm as described above, an excessive stress is applied to the toner. Therefore, a deterioration of the toner, such as a separation or a depression of an external additive included in the toner, may occur and the image developed on the photosensitive medium may be deteriorated. Also, the developing roller 5 may be worn by a high tip pressure thus shortening its life time.
Also, the toner layer formed on the developing roller 5 may be non-uniform if the tip pressure of the regulating blade is decreased while maintaining the regulating angle θ1 to be about 24°. Accordingly, the toner supplied to the photosensitive medium 1 may be insufficiently charged at a predetermined area, so that the toner may be developed at an unwanted position on the photosensitive medium.